Field of the Invention
This invention relates to a method of reading glass dosimeters for use radiation dosimetry and the like and an apparatus for reading the glass dosimeter, and in particular, to a method and an apparatus for reading the radiation dose of a fluorescent glass element at a high sensitivity and with high accuracy.
Description of the Prior Art
For radiation protection monitoring, the operators of a nuclear reactor, an accelerator, an X-ray generator or a radioisotope device must carefully control radiation, and it is necessary for the operators handling such radiation generating systems to measure their individual doses. Thus, it has become very important to read the exposure dose of every dosimeter separately and control the dose data properly.
When a conventional radiophoto-luminescence dosemeter is used, a glass element holder 2 in which a fluorescent glass element 1 is mounted is inserted into a lower case 3 as shown by an arrow A in FIG. 8, and then the lower case 3 is installed in an upper case 4 as shown by an arrow B. The fluorescent glass element 1 is transported to, or installed in, a proper place in a so-called capsulized state. At the time of reading the exposure dose of the fluorescent glass element 1, the glass holder 2 is taken out from the upper and lower cases 4 and 3, and is disposed in a predetermined reading position. Thereafter, the read-out is carried out for the exposure dose received by fluorescent glass element 1.
As shown in FIG. 9, when exciting ultraviolet rays 5 are transmitted to the fluorescent glass element 1 after exposure to the radiation, red-orange light of radiophoto luminescence 6 (hereinafter called as RPL) corresponding to the exposure dose is emitted from a fluorescent light detecting surface which is oriented in a direction different from the direction of the ultraviolet rays transmitted to the fluorescent glass element. The exposure dose of the fluorescent glass element 1 is measured by reading the RPL.
The fluorescent light 6 emitted from the fluorescence light detecting surface of the fluorescent glass element 1 after the exciting ultraviolet rays 5 have been supplied thereto comprises a fluorescent light component generated by the exposure to UV radiation and a fluorescent light component (predose component) which is intrinsic to the fluorescent glass element 1 and exists without the exposure to radiation. Thus, in order to obtain the true radiation induced RPL, the amount of the predose component must be subtracted from the total amount of the fluorescent light generated in the fluorescent glass element 1.
In view of the fact that the decay time constants of RPL are greatly different from each other, the inventors of this invention have proposed a method and an apparatus for reading only the RPL efficiently (as described in Japanese Unexamined Patent Application Publication No. Sho 59-190681), and for reading RPL by delaying the impressing time of the dynode voltage of a multiplier and suppressing the detecting sensitivity of the predose (basedose) which decay more quickly (as described in Japanese Unexamined Patent Application Publication No. Sho 61-292582).
After various studies of the measurement of radiation dose, the following problems were pointed out:
(1) One problem exists in the predose. The decay of one predose component is slower than that of the other predose component. Even if the detecting sensitivity of the predose component having the faster decay is suppressed, the effect of the predose component having the slower decay remains, since this slower predose component is fairly larger than RPL emitted from low radiation dose. In other words, it is very difficult to detect the RPL with a high sensitivity and high accuracy by suppressing the detecting sensitivity for the predose component having the faster decay, which is a part of the fluorescent light 6 generated in the fluorescent glass element 1 excited by the ultraviolet rays, and by subtracting the predose having the slower decay.
(2) Another problem exists in the intensity distribution of fluorescent light. With the conventional method, exciting ultraviolet pulses produced by a nitrogen gas laser unit are emitted to a fluorescent glass element 1 when the deviation of the intensities of the exciting ultraviolet pulses changes the intensities of the fluorescent pulses generated in the fluorescent glass element. A part of the exciting ultraviolet pulses are emitted to a reference fluorescent glass element. After the deviations of the intensities of the fluorescent pulses produced in the fluorescent glass element 1 exposed to radiation and the reference fluorescent glass element have been obtained, the intensity of the ultraviolet pulses is corrected on the basis of the intensity deviation of the reference glass.
When the exciting ultraviolet light pulses are separated and parts thereof are supplied to the reference fluorescent glass element, however, the intensity ratios of the ultraviolet pulses transmitted to the fluorescent glass element exposed to radiation and the reference fluorescent glass element vary because the intensity distribution in a plane perpendicular to the direction of the exciting ultraviolet pulses are different depending on the distances from the nitrogen gas laser unit. Thus, it is also difficult to read the intensity of the fluorescent light at a high accuracy.
(3) A further problem resides in the correction of the decay time constant of the predose or basedose components. Since the predose component having a slower decay is detected at a delayed sampling time, the intensity of this predose is decayed in accordance with the decay time constant. To compensate for the decay, it is necessary to multiply the read-out fluorescent intensity by a correction factor in order to find the exposure dose of the fluorescent glass element. Since, however, each fluorescent glass element may have a different decay time constant, the correction factors of fluorescent glass elements must be carefully considered when the radiation exposure is determined.
It is preferable that the correction factors of each fluorescent glass element should be separately determined. However, it is very complicated as the operator must set and input the correction factors for each fluorescent glass element, and the correction factors corresponding to the respective fluorescent glass elements must be separately selected at a time of continuous measurement. In order to avoid this complicated process, a representative constant is selected as the correction factor, resulting in lowering the measurement accuracy. This is also true for the correction of the sensitivity of the fluorescent glass elements.
This invention was made in view of the above situation and provides a method and an apparatus for reading the exposure dose of fluorescent glass element at a high sensitivity and at a high accuracy by picking up reduced amounts of the predose components of a fluorescent glass element.
Another object of this invention is to provide an apparatus for reading exposure dose by detecting the fluorescent intensity from a fluorescent glass element at a high accuracy without the influence of the errors of the exciting ultraviolet intensity distribution.
A further object of this invention is to provide an apparatus in which the correction factors of individual fluorescent glass elements are suitably determined, the exposure dose of every fluorescent glass element is obtained, and the reliability of the radiation exposure determination is enhanced.